The present invention relates to terpolymers of a vinyl aromatic monomer such as styrene, .alpha.-methylstyrene, chlorostyrene and the like; an unsaturated nitrile monomer such as acrylonitrile, methacrylonitrile and the like and a dialkylvinyl aromatic phosphonate and to miscible blends thereof with certain polymers such as poly(styrene acrylonitrile) polymers (SAN) and polymers and copolymers of methylmethacrylate.